I'm using early 340 manifolds, but i won't be buying one of those reproduction heat shields. What's the next best thing for heat protection? Do those fiberglass tubes work well? I suppose some are better than others.

Yes, those tubes work well for that.

I have been using them for years without any issues. They are lying right on the headers and have held up well.

In my experiences, if you are using a good quality wire and they are routed to stay off the manifold(s) you shouldn't have an issue.
My experiences with customers cars differ than the others on the longevity of the fiberglass shields. I have seen many cooked wires with the shields on them. IMO properly routed wires should not need anything to shield them from heat. In addition they look nicer

This was my MacGyver solution, wire loom and tie straps.